1. Field of the Invention
The present invention is in the field of power converters. The present invention is further in the field of semiconductor switching power converters. The present invention further relates to the field of integrated synthetic ripple control methods for switching power converters and circuits. The present invention is further in the field of integrated switching power converters. The present invention is further in the field of hysteretic control types for switching power converters. The present invention is further in the field of single inductor multiple output switching power converters. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Modern electronic applications require power management devices that supply power to integrated circuits or more generally to complex loads. In general, power switching converters are becoming more and more important for their compact size, cost and efficiency. Switching power converters comprise isolated and non isolated topologies. The galvanic isolation is generally provided by the utilization of transformers.
Modern switching power converters are in general divided in step-down power converters, also commonly known as “buck converters”, and step-up power converters commonly known as “boost converters”. This definition stems from the ability of the converter to generate regulated output voltages that are lower or higher than the input voltage regardless of the load applied.
One class of modern switching power converters implemented in integrated circuits is the one comprising hysteretic control or pseudo-hysteretic control where a synthetic ripple signal is generated and compared to a reference to determine the duty cycle of the switching period in order to regulate the output voltage at the desired level. These hysteretic power converters do not include an error amplifier, a specific compensation network or a periodic signal to determine the switching frequency.
In fact their switching frequency is determined by several factors like the input voltage, the output voltage, the load, the output capacitor value, the inductor value, the hysteresis value, and the general propagation delays of the feedback network, of the comparator, of the driver, and of the output stage. However frequency control circuits are commonly implemented in order to control the frequency.
When the load currents are not very high, in order to reduce the number of external passive components, in particular the inductors, the same switching power converter can be configured to serve multiple different loads. In these configurations, multiple output power switches are used to select to which output of the converter, the energy (stored in the inductor) should be diverted at any time. Typically, since the output regulated voltages and the loads may be different from one another, only one output power switch is turned on at any given time and the transition to shift energy from one output to the other has to be performed making sure that no cross conduction is ever occurring.
This class of converters includes bucks (where all the outputs are regulated at a voltage lower than the input voltage), boosts (where all the outputs are regulated at a voltage higher than the input voltage), and buck-boosts (where the outputs may be regulated at voltages higher or lower than the input voltage) and hybrid configurations (where some outputs are regulated at voltages higher than the input voltage and some other at a voltage lower than the input voltage). In the case of the buck power converters there is an extra switch in the current path with respect to the more conventional approach of a power converter with a single output, while in the case of the boost power converters the number of the switches in the current path can be the same as for the more traditional boost power converter.
The main advantage of these converters, known as SIMO (Single Inductor Multiple Output) power converters is the reduction of the board space and associated cost due to the removal of the power inductors. In reality, a reduction of the EMI (Electro-Magnetic Interference) is also possible depending on the implementation adopted. Depending on the specific type of power converter another advantage is the reduction of power switches (more evident in the case of a SIMO boost) and of control circuitry. The most obvious disadvantage is that in some cases the efficiency of the converter may be lower with respect to the more conventional case. In particular for the case of the buck SIMO, the extra switch on the current path introduces a loss that is not present in the conventional buck converter.
Furthermore the current in the inductor is the sum of the load currents (in a SIMO buck converter) and since the DC losses, in the switching power transistor and in the inductor series resistance, are proportional to the square of the current, the losses are increased by the fact that the inductor is serving more outputs. For these reasons it is reasonable to assume that the advantage of the SIMO converter is more evident for low and middle level load currents in which case the efficiency can easily reach values in the order of 90%.
Another disadvantage is that, by serving multiple outputs, the output voltage ripple is typically higher than in the more conventional approach. In this case a compromise between the size of the output capacitors, the frequency of multiplexing the energy between the various outputs, and the number of outputs to be served may attenuate the effects of this drawback. Of course the problem is more severe for a large number of outputs.
A further disadvantage may be represented by the poor response to a load transient. This parameter is particularly important nowadays since the loads of the power converters are other integrated circuits that are periodically turned on and off quite frequently. The ability to respond very swiftly to a load change maintaining the minimum voltage droop or limiting the over-voltages is a very important parameter. If the power converter has to serve multiple outputs it is clearly more difficult to promptly respond to asynchronous load changes.
In this class of power converters two separate control loops can be identified. The first control loop is the one that determines the duty cycle and therefore the amount of energy to be stored in the inductor at any given time. This control loop is present in any type of switching power converter. The second control loop is the one that determines how to share the energy between the various outputs. This second loop is not present in the conventional switching power converters.
Many techniques have been developed to more effectively deal with these two control loops. There are several difficulties to overcome in operating Single Inductor Multiple Output converters. The first one is the so called “cross-regulation” which refers to the perturbations in the loops that affect one or more of the outputs when the one output is subject to a load transient. Ideally the power converter should operate to respond to the load transient of one of the outputs without affecting the other outputs. However this is very difficult to achieve because a load transient changes the level of energy in the inductor and that may be reflected onto all the outputs.
A second difficulty for these converters is the operation when the level of loads between different outputs is very dissimilar. For simplicity let us consider the case of a SIDO (Single Inductor Dual Output) where only two outputs are present and their loads are very different. In this case the energy in the inductor is adequate for one of the outputs and too large for the other one. This may cause loss of regulation, instability in the loops or very high voltage ripple for one of the outputs.
A third obstacle to be overcome for this type of power converter is the case in which one of the outputs supports a very light load while the other has a heavy load. In theory the light load output would require the converter to operate in DCM (Discontinuous Conduction Mode) while the other output would require operation in CCM (Continuous Conduction Mode). DCM makes reference to the discontinuous nature of the inductor current, which occurs when the current falls to zero at each period.
One of the solutions widely adopted to minimize the cross conduction is depicted in the block diagram of FIG. 1 for the case of a SIDO power converter. The transistors M1 and M2 are power devices whose duty cycle determines the energy to be stored at each cycle in the inductor L1. The power devices M3 and M4 represent the switches that divert the energy to either the output 1 or to the output 2. The control loop to drive the transistors M1 and M2 is conventional. The error amplifier 3 feeds the amplified error signal, generated by the difference between a signal representative of one of the outputs and a reference voltage to a comparator 1. The comparator 1 compares this amplified error signal to a ramp signal in order to determine the duty cycle.
Similarly, the error amplifier 4 and the comparator 5 generate the signal that drives the drivers block 6 to determine which of the power transistors M3 or M4 should be turned on to divert the energy flow to the corresponding output. In this configuration one of the outputs (output 2) is controlling the amount of energy in the inductor and the other output (output 1) is controlling the multiplexing of the energy to the outputs. This technique is simple and quite effective provided that the system is made stable, but its big drawback is that if the load at the output governing the amount of energy to be stored in the inductor drops significantly, the other output cannot be regulated anymore. On the other hand if both loads are large enough (both in CCM) this approach reduces significantly the cross regulation.
The prior art Lenk (U.S. Pat. No. 6,222,352) is an example of what described above where two separate loops are generated and where one output voltage governs the regulation of the inductor current and the other output voltage governs the sharing of the energy between the two outputs. In addition Lenk teaches the use of the body diode of the output power transistors to smoothly transition the transfer of energy from one output to the other.
Ivanov (U.S. Pat. No. 6,522,110) describes a single inductor multiple output buck-boost converter with general unspecified control topology. May (U.S. Pat. No. 6,977,447) reports a single inductor dual output boost power converter, where a load select module and a feedback module feed instantaneous information to a regulation module in order to process the relative information to operate the power converter.
The prior art Kranz (U.S. Pat. No. 7,176,661) is another example of a single inductor dual output boost power converter, where one output voltage signal is processed to control and modulate the energy stored in the inductor at any one time and one output voltage signal is processed to control the multiplexing of the energy between the two outputs. More interesting appears the approach taken by Chen et al. (U.S. Pat. No. 7,224,085) who describe a non conventional single inductor dual output buck converter with only three switches, where the energy stored in one of the outputs is used to provide power to the second output. However the output voltage ripple and the efficiency offered by this solution are not optimum.
Easwaran et al. (U.S. Pat. No. 8,049,472) describes a complex system to control a single inductor multiple output buck power converter where a series of error signals are generated and processed to control the energy into the inductor and the time division utilized for the multiplexing of the output switches.
However all the cited prior art does not describe a cost effective, accurate and simple method for achieving single inductor multiple output power conversion with optimum transient performance maintaining stability in all conditions. It is therefore a purpose of the present invention to describe a novel single inductor multiple output switching power converter that combines the characteristic of being simple, cost effective, having minimum cross regulation in all load conditions, excellent load transient performance and minimum size passive components.
It is another purpose of the present invention to describe a power converter that can exhibit minimum output voltage ripple. It is another purpose of the present invention to describe a constant frequency single inductor multiple output power converter that is stable and can operate at high switching frequencies.